We're going to GC during our Las Vegas trip this summer { not the Skywalk } . I don't know of any discounted tours but we always use "Las Vegas Grand Canyon Tours" { link is below }. IMO they're reasonably priced and offer great tours. We've tried other tour companies in the past but once again IMO this company is great.

http://www.lasvegasgrandcanyontour.com/skywalk.htm

Victor